High vacuum pump with cryosorption pumping element



June 11, 1968 R. HECHT 3,387,767

HIGH VACUUM PUMP WITH CRYOSORPTION PUMPING ELEMENT Filed Dec. '7, 1966Unite ABSTRACT OF THE DISCLOSURE A cryosorption vacuum pump element witha porous mass of sintered fibers forming tunnels and dispersed sorbentpowders in the tunnels.

The present invention relates to vacuum pumps of the cryosorption type;that is, pumps with a pumping element containing sorbent such asactivated charcoal or natural or synthetic zeolite.

Background of the invention The cryosorption vacuum pump has been knownsince used as a rough pump by Sir James Dewar and Thomas Edison, circa1875. This form of usage continues to the present day; e.g., Grant etal., Review of Scientific Instruments, May 1963, pp. 587-8. In recentyears there has been a great deal of interest in the usage ofcryosorption pumps for the high vacuum range.

Cryosorption pumps, like cryogenic pumps, afford the advantage over ionand diffusion pumps of freedom from electrostatic and magnetic fieldsand freedom from pump generated hydrocarbons such as methane or pumpoil. Cryosorption pumps afford the advantage over cryogenic pumps oftrapping high vapor pressure gases such as nitrogen at 77 K. and underhigh vacuum, hydrogen at K., etc., that would escape a cryogenic pump.Cryosorption pumping elements can be used in separate pump bodies or gaspumping elements in, say, an environmental test chamber which may hethought of as a pump for purposes of this application.

However, the use of cryosorption in high vacuum pumping requires aneffective sorbent mounting arrangement. A cryosorption pumping elementmust withstand cycling over a substantial temperature range-operating asa pump at 77 or 20 or even 4.2 K. and then baking out at about 573 K.between pumping cycles to regenerate the sorbent. Good bonding of thesorbent to the pumping element is necessary to withstand these wideswings of temperature. Good bonding is also necessary to provideeffective heat transfer within the pumping element. Other desiderata ofcryosorption pumping elements are freedom from organic components suchas conventional epoxy binders, ruggedness, economy of manufacture andexposure of substantially all the contained sorbent to the gas to bepumped.

Object It is the object of the invention to provide a new cryosorptionpumping element for use in high vacuum pumps which affords compatibilitywith the high vacuum environment and high pumping speed as well asruggedness, economy and good bonding of its contained sorbent. Otherobjects, features, and advantages will be stated in or will be apparentfrom the following description of the invention.

General description In accordance with the foregoing objects, a newcryosorption pumping element is provided for high vacuum pumps. Theelement comprises a sintered mass of metallic fibers and sorbentpowders. This combination offers States Patent O improvement over theprior art in several respects. It provides excellent thermalconductivity for a cryosorptron pump element, the conductivity throughthe metal fiber mass being 520% of that of the same volume of solidmetal. The volume of less conductive sorbent is dispersed by its powderform; the sorbent itself does not substantially impede the necessaryheat transfer as in prior art arrangements. This combination also offersa unique interaction of its components. The sintered mass of metalfibers provides a porous structure with tunnels leading to the sorbentpowder within. The walls of these tunnels are the metal fibers whichtend to wick the gas being pumped. That is a statistical majority of gasmolecules striking the metal fiber with residual energy preferentiallyglide along the fiber surface rather than leaving the fiber surface.They glide until they are conducted to sorbent particles where they arepumped by entry into the pores of the sorbent. Some gas moleculesstriking the metal fibers are cryopumped or cryosorbed by the porousmetal fiber mass itself. Furthermore, the use of dispersed powderedsorbent as opposed to bulk sorbent or sintered powder sorbent provides amaximum ratio of external sorbent surface to total sorbent withconsequent increase in both pumping speed and total gas capacity.

Specific description The invention has been generally described above. Apreferred embodiment is now described specifically with reference to theaccompanying single figure of drawing which is a cross-sectionalrepresentation of a portion of a cryosorption element according to apreferred embodiment of the invention.

The cryosorption pumping element 10 has a plate form with forward andreverse surfaces F and R. A coolant pipe 12 carrying a cryogenic coolant14 such as liquid nitrogen is brazed to the element 10. The elementitself is formed of a felted or woven mass of metal fibers 16,preferably copper. The fibers make random contacts with each other andare sintered at these contact points 18. Methods of making such porousmetal fiber bodies are disclosed, for instance, in US. Patents 2,903,787and 3,178,280. The properties of a fiber system such as that shown inUS. Patent 3,178,280 are given in Materials and Design Engineeringmagazine, October 1964 issue.

In the present combination, the fiber metal system is arranged toprovide a density between 20 to 30% of equivalent solid metal. The metalfibers are on the order of about 10 to microns (.004 inch) in diameterand about to several inches in length. The mass is constructed toprovide a median tunnel diameter on the order of 10-20 microns. Sorbentpowders 20 such as Linde l3 synthetic zeolite having an average diameterof 10 microns are incorporated in the fiber metal. In this combinationthe fibers form intersecting tunnels 22 leading to the sorbent powderstherein.

The fiber mass is prepared with an inner layer A containing the sorbentpowders and outer layers B, C free of sorbent powders. The outer layersprevent migration of the sorbent powders out of the pumping element. Thethickness of the element 10 is about /8 to /2 inch. In brazing thecopper pipe 12 to the element 10, the local surface 24 of the element isground to weld the fibers together at the contact point, thus providinga barrier to prevent the braze alloy from wicking into the pumpingelement.

While the invention has been described in terms of its principal usageas a high vacuum pump, it should be understood that other applicationsare feasible. For instance, a canister can be provided with a stack ofplates 10 mounted on a chilled support, or a stack of plates 10 brazedto copper plates which are mounted on a chilled support, for use as aroughing pump. A plate 10 can be mounted as a bafile above the inlet ofa difiusion pump or ion pump and used with or Without chilling to trapbackstreaming gas molecules from the pump and, with cryogenic cooling,to pump the system through the intermediate range of vacuum betweenrough vacuum; e.g., (10 microns and above) and high vacuum (.1 micronand below).

Several variations can also be made in the cryosorption plate 10 itselfwithin the scope of the invention. The term plate as used herein refersto various thin wall members such as cylinders or fiat plates. Thesorbent metal composite can be formed in a wide variety of curvedshapes. Other permissible variations would include the substitution ofother conductive metals and sorbents for the copper and metal silicatedisclosed above. A solid metal backing plate can be clamped or brazed t0the fiber structure of the invention and the density of fibers can begraded to be heaviest adjacent the backing plate and lightest away fromthe backing to achieve an optimum combination of heat transfer andporosity properties for vacuum pumping.

It is therefore intended that the above description shalle be read asillustrative and not in a limiting sense.

What is claimed. is:

1. A high vacuum pump containing a composite sorbent-metal pumpingelement and means for cooling the pumping element to the cryogenictemperature range, the pumping element comprising a porous sintered massof crossing metal fibers secured to each other at random points ofcontact by intermetallic bonds to form a network of intersecting tunnelsand dispersed sorbent powders disposed in said tunnels.

2. A sorbent-metal composite for cryosorption pumping and the likecomprising a porous sintered mass of crossing metal fibers secured toeach other at random points of contact by intermetallic bonds'to form anetwork of intersecting tunnels and dispersed sorbent powders disposedin said tunnels.

3. The sorbent-metal composite of claim 2 in plate form.

4. The sorbent-metal composite of claim 3 with the two outer layerportions of the plate being essentially free of sorbent powder in thesaid tunnels.

5. The sorbent-metal composite of claim 2 with an outer layer of thefiber mass being essentially free of sorbent powders in the saidtunnels.

References Cited UNITED STATES PATENTS 3,147,910 9/1964 Iepsen 230-693,241,740 3/1966 Hamilton 23069 ROBERT M. WALKER, Primary Examiner.

